Method for removing single ring aromatic hydrocarbons

ABSTRACT

A method of removing at least one single ring aromatic hydrocarbon from a hydrocarbon contaminated fluid. The method includes contacting the hydrocarbon contaminated fluid with carbon nanotubes to adsorb the at least one single ring aromatic hydrocarbon while exposing the hydrocarbon contaminated fluid and the carbon nanotubes to UV irradiation from at least one UV light source, preferably a UV light emitting diode (LED), with a wavelength of about 315-415 nm, preferably about 365 nm, to form a treated fluid having a reduced concentration of the at least one single ring aromatic hydrocarbon relative to the hydrocarbon contaminated fluid.

BACKGROUND OF THE INVENTION

Technical Field

The present disclosure relates to methods of removing single ringaromatic hydrocarbons, such as para-xylene, from a hydrocarboncontaminated fluid. More specifically, the present disclosure relates toa method of removing single ring aromatic hydrocarbons from ahydrocarbon contaminated fluid using carbon nanotubes to adsorb thesingle ring aromatic hydrocarbons together with UV irradiation,preferably from a UV light emitting diode (LED), to oxidize and degradethe single ring aromatic hydrocarbons in the hydrocarbon contaminatedfluid and adsorbed on the carbon nanotubes.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, is neitherexpressly nor impliedly admitted as prior art against the presentinvention.

Single ring aromatic hydrocarbons, such as benzene, toluene,ethylbenzene, and the three isomers of xylene i.e. para-xylene,meta-xylene, and ortho-xylene, or BTEX, are petroleum hydrocarbons. Forexample, BTEX constitutes about 18 wt % of total hydrocarbons ingasoline. Compared with aliphatic hydrocarbons, aromatic hydrocarbons,including single ring aromatic hydrocarbons, are more toxic. Forexample, inhaling para-xylene can cause dizziness, headache, drowsiness,and nausea. Long term exposure to para-xylene can cause developmentaldefects and damage the central nervous and reproductive systems. Thus,single ring aromatic hydrocarbons pose a serious health risk to humanswhen they enter into soil and groundwater due to, for example, leakingoil tanks, blowouts, and improper discharge of petroleum hydrocarbonsand untreated waste water from the petro-chemical industry. Conventionalmethods for removing single ring aromatic hydrocarbons includecoagulation and flocculation, however, they are often ineffective.

It is thus an object of the present disclosure to provide a simple, andyet efficient and effective method for removing single ring aromatichydrocarbons from a hydrocarbon contaminated fluid, such as apara-xylene containing aqueous solution, using a combination ofadsorption with carbon nanotubes and UV irradiation.

BRIEF SUMMARY OF THE INVENTION

The present disclosure relates to a method of removing at least onesingle ring aromatic hydrocarbon from a hydrocarbon contaminated fluid.The method includes contacting the hydrocarbon contaminated fluid withcarbon nanotubes to adsorb the at least one single ring aromatichydrocarbon while exposing the hydrocarbon contaminated fluid and thecarbon nanotubes to UV irradiation from at least one UV light sourcewith a wavelength of about 315-415 nm to form a treated fluid having areduced concentration of the at least one single ring aromatichydrocarbon relative to the hydrocarbon contaminated fluid.

In one or more embodiments, the at least one single ring aromatichydrocarbon is at least one selected from the group consisting ofbenzene, toluene, ethylbenzene, ortho-xylene, meta-xylene, para-xylene,cumene, and styrene, and derivatives thereof.

In one or more embodiments, the hydrocarbon contaminated fluid is anaqueous solution, the at least one single ring aromatic hydrocarbon ispara-xylene, and the method reduces the concentration of para-xylene inthe aqueous solution by at least 30% in less time than a substantiallysimilar method without contacting the aqueous solution with the carbonnanotubes.

In one or more embodiments, the hydrocarbon contaminated fluid is anaqueous solution, the at least one single ring aromatic hydrocarbon ispara-xylene, and the method reduces the concentration of para-xylene inthe aqueous solution by at least 30% in less time than a substantiallysimilar method without exposing the aqueous solution and the carbonnanotubes to the UV irradiation.

In one or more embodiments, the hydrocarbon contaminated fluid is anaqueous solution, the at least one single ring aromatic hydrocarbon ispara-xylene, and the reduced concentration of the para-xylene in thetreated fluid is at least 95% reduced relative to the hydrocarboncontaminated fluid.

In one or more embodiments, the carbon nanotubes comprise unmodifiedcarbon nanotubes, modified carbon nanotubes, or a combination ofunmodified carbon nanotubes and modified carbon nanotubes.

In one or more embodiments, the carbon nanotubes have an outer diameterranging from about 8 nm to 80 nm.

In one or more embodiments, the amount of the carbon nanotubescontacting the hydrocarbon contaminated fluid ranges from about 0.2 g/Lto 1 g/L of the hydrocarbon contaminated fluid.

In one or more embodiments, the hydrocarbon contaminated fluid has a pHof about 6-8 before the contacting and the exposing.

In one or more embodiments, the at least one UV light source comprises alight emitting diode (LED).

In one or more embodiments, the hydrocarbon contaminated fluid iscontacted with the carbon nanotubes while the hydrocarbon contaminatedfluid and the carbon nanotubes are exposed to the UV irradiation at atemperature of about 20-30° C. and a pressure of about 0.5-1.5 bar.

In one or more embodiments, a plurality of the carbon nanotubes is in apowder form.

In one or more embodiments, the contacting forms adsorbed carbonnanotubes comprising at least one single ring aromatic hydrocarbon andthe method further comprises removing the adsorbed carbon nanotubes fromthe treated fluid.

In one or more embodiments, the contacting forms adsorbed carbonnanotubes comprising at least one single ring aromatic hydrocarbon andthe method further comprises removing the adsorbed carbon nanotubes fromthe treated fluid and regenerating the adsorption ability of theadsorbed carbon nanotubes.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a graphical presentation of the percentage of p-xylene removedfrom an aqueous solution with time by adsorption with carbon nanotubes(CNTs) only, by UV irradiation only with a UV LED, or by a combinationof adsorption with carbon nanotubes and UV irradiation with a UV LEDaccording to Example 2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure provides a method of removing at least one singlering aromatic hydrocarbon from a hydrocarbon contaminated fluid. Themethod includes contacting the hydrocarbon contaminated fluid withcarbon nanotubes to adsorb the at least one single ring aromatichydrocarbon while exposing the hydrocarbon contaminated fluid and thecarbon nanotubes to UV irradiation from at least one UV light sourcewith a wavelength of about 315-415 nm to form a treated fluid having areduced concentration of the at least one single ring aromatichydrocarbon relative to the hydrocarbon contaminated fluid.

Non-limiting examples of the at least one single ring aromatichydrocarbon include benzene, toluene, ethylbenzene, ortho-xylene,meta-xylene, para-xylene, cumene, and styrene, and their respectivederivatives. Toluene, ethylbenzene, ortho-xylene, meta-xylene,para-xylene, cumene, and styrene are in fact hydrocarbon derivatives ofbenzene. Other derivatives of benzene removable by the disclosed methodmay include, without limitation, halogen-containing benzene derivatives,e.g. fluorobenzene, chlorobenzene, bromobenzene, and iodobenzene;oxygen-containing benzene derivatives, e.g. phenol, benzoic acid,benzaldehyde, acetophenone, methyl benzoate, and anisole;nitrogen-containing benzene derivatives, e.g. aniline, nitrobenzene,benzonitrile, and benzamide; and sulfur-containing benzene derivatives,e.g. benzenesulfonic acid. Likewise, halogen-containing derivatives,hydrocarbon derivatives, oxygen-containing derivatives,nitrogen-containing derivatives, and sulfur-containing derivatives oftoluene, ethylbenzene, ortho-xylene, meta-xylene, para-xylene, cumene,and styrene may also be removed by the disclosed method.

In one embodiment, the hydrocarbon contaminated fluid is a single ringaromatic hydrocarbon containing liquid, such as an aqueous solution. Forexample, the hydrocarbon contaminated fluid may be at least one selectedfrom the group consisting of a refinery industry effluent, a chemicalindustry effluent, waste water, ground water, river water, lake water,water that has been contacted with sediment and soil contaminated withsingle ring aromatic hydrocarbons.

In another embodiment, the hydrocarbon contaminated fluid is a singlering aromatic hydrocarbon containing gas. In still another embodiment,the hydrocarbon contaminated fluid is a mixture of at least one singlering aromatic hydrocarbon containing liquid and at least one single ringaromatic hydrocarbon containing gas.

In some embodiments, the hydrocarbon contaminated fluid has a pH of1-14, 3-12, preferably 3-10, preferably 5-10, or more preferably 6-8before the contacting and exposing. In some embodiments, the hydrocarboncontaminated fluid pH is adjusted such that the single ring aromatichydrocarbon(s) are positively charged, for example, the single ringaromatic hydrocarbons have one or more protonated amine groups, whereasthe carbon nanotubes, particularly the modified carbon nanotubes, forexample, with carboxylate and/or phosphate groups, have a negativesurface charge (typically at a neutral or basic pH) to achieve a higheradsorption efficiency due to a stronger electrostatic interactionbetween the single ring aromatic hydrocarbon(s) and the surface of thecarbon nanotubes.

The carbon nanotubes may be single-walled carbon nanotubes and/ormulti-walled carbon nanotubes. In some embodiments, the multi-walledcarbon nanotubes may have a structure conforming to the Russian Dollmodel, i.e. they contain sheets of graphite arranged in concentriccylinders. In other embodiments, the multi-walled carbon nanotubes havea structure conforming to the Parchment model, i.e. they contain asingle sheet of graphite rolled in around themselves and resemble ascroll of parchment.

In some embodiments, the carbon nanotubes have an outer diameter ofabout 1-100 nm, preferably about 5-90 nm, preferably about 8-80 nm,preferably about 10-70 nm, preferably about 15-60 nm, or more preferablyabout 20-50 nm, or more preferably 30-40 nm.

In one embodiment, the carbon nanotubes are unmodified, or raw, carbonnanotubes. In another embodiment, the carbon nanotubes are modified orfunctionalized carbon nanotubes that preferably have an enhancedadsorption capacity for the single ring aromatic hydrocarbon(s).Non-limiting examples of such modified carbon nanotubes includemulti-walled carbon nanotubes oxidized by NaOCl, disclosed by ChinesePatent No. CN102120177 B, and incorporated herein by reference in itsentirety; multi-walled carbon nanotubes activated by KOH, disclosed byFei Yu, Yanqing Wu, Xiaoman Li, and Jie Ma, Kinetic and ThermodynamicStudies of Toluene, Ethylbenzene, and m-Xylene Adsorption from AqueousSolutions onto KOH-Activated Multiwalled Carbon Nanotubes, J. Agric.Food Chem., 2012, 60 (50), pp 12245-12253, and incorporated herein byreference in its entirety; and single-walled carbon nanotubes treatedwith nitric acid, disclosed in Chin, Ching-Ju Monica; Shih, Li-Chieh;Tsai, Hen-Je; Liu, Ta-Kang, Adsorption of o-xylene and p-xylene fromwater by SWCNTs, CARBON, 45, (6), pp 1254-1260, 2007/05, andincorporated herein by reference in its entirety. In still anotherembodiment, the carbon nanotubes are a combination of unmodified carbonnanotubes and at least one type of modified carbon nanotubes. The massratio of the unmodified carbon nanotubes to the at least one type ofmodified carbon nanotubes, and the mass ratio among different types ofthe modified carbon nanotubes may vary without limitation, depending onthe availability and cost considerations of the unmodified versus themodified carbon nanotubes and whether the conditions of the hydrocarboncontaminated fluid, e.g. the fluid pH and the initial concentration ofthe single ring aromatic hydrocarbon(s) in the fluid, favor and/ornecessitate the adsorption by the modified carbon nanotubes, etc. In apreferred embodiment, the conditions of the fluid, for example, the pHof the fluid, are adjusted for the maximal adsorption by the modifiedcarbon nanotubes, and the mass ratio of the modified carbon nanotubes tothe unmodified carbon nanotubes is at least 1:1, more preferably from1:1 to 10:1, since the modified carbon nanotubes generally have a higheradsorption capacity for the single ring aromatic hydrocarbons than theunmodified carbon nanotubes. Additionally, when the unmodified carbonnanotubes are combined with the modified carbon nanotubes, theunmodified carbon nanotubes may form a separate layer or section fromthe modified carbon nanotubes, for example, in a fixed bed reactor, orthe unmodified carbon nanotubes and the modified carbon nanotubes mayfreely mingle with each other, for example, in a fixed bed reactor, in abatch adsorption vessel, or in a fluidized bed reactor.

The UV irradiation, preferably in the presence O₂, oxidizes and degradesthe single ring aromatic hydrocarbons both in the hydrocarboncontaminated fluid and adsorbed on the carbon nanotubes by converting O₂to O₃ that in turn produces highly reactive hydroxyl radicals. The O₂may be from atmospheric air and/or other sources, such as an O₂ gastank, from which O₂ may be pumped or injected into the fluid.

Some oxidation and degradation products of the single ring aromatichydrocarbons by the UV irradiation may be oxygen-containing derivativesof the single ring aromatic hydrocarbons that include, withoutlimitation, phenol, benzoic acid, benzaldehyde, acetophenone, methylbenzoate, and anisole. Other oxidation and degradation products of thesingle ring aromatic hydrocarbons may include deep oxidation productssuch as carbon dioxide and water. For example, Perumal Devaraji, NaveenK. Sathu, and Chinnakonda S. Gopinath showed UV oxidation of benzene tophenol in “Ambient Oxidation of Benzene to Phenol by Photocatalysis onAu/Ti_(0.98)V_(0.02)O₂: Role of Holes,” ACS Catal., 2014, 4 (9), pp2844-2853, incorporated herein by reference in its entirety. For anotherexample, UV irradiation can oxidize m-xylene to form benzaldehyde,acetophenone, benzoylformic acid, benzoic acid, CO₂, and water, asdisclosed by Yi-Ting Wu, Yi-Hui Yu, Van-Huy Nguyen, Kung-Te Lu, JeffreyChi-Sheng Wu, Luh-Maan Chang, Chi-Wen Kuo, Enhanced xylene removal byphotocatalytic oxidation using fiber-illuminated honeycomb reactor atppb level, Journal of Hazardous Materials 262 (2013) 717-725,incorporated herein by reference in its entirety. According to thedisclosed method, all or a portion of the oxidation and degradationproducts of the single ring aromatic hydrocarbons may be advantageouslyadsorbed by the carbon nanotubes (raw and/or modified) and/or otheradsorbents that can be included in additional to the carbon nanotubes asdescribed below. For example, carbon nanotubes, preferably carbonnanotubes impregnated with aluminum oxide or ferric oxide, canefficiently remove phenol from an aqueous solution, as disclosed byIhsanullaha, Hamza A; Asmalya, Tawfik A; Salehb, Tahar Laouie, VinodKumar Gupta, and Muataz Ali Atiehd, Enhanced adsorption of phenols fromliquids by aluminum oxide/carbon nanotubes: Comprehensive study fromsynthesis to surface properties, Journal of Molecular Liquids, Volume206, June 2015, Pages 176-182; and by Hamza A. Asmalya, Basim Abussaudb,Ihsanullahb, Tawfik A. Salehc, Vinod Kumar Guptad, and Muataz Ali Atieh,Ferric oxide nanoparticles decorated carbon nanotubes and carbonnanofibers: From synthesis to enhanced removal of phenol, Journal ofSaudi Chemical Society, Volume 19, Issue 5, September 2015, Pages511-520; each incorporated herein by reference in its entirety.

In one embodiment, the at least one UV light source comprises a mercurylamp. In a preferred embodiment, the at least one UV light sourcecomprises a light emitting diode (LED). Compared with a mercury UV lamp,a UV LED has a narrower spectral output centered around a specificwavelength, e.g. ±10 nm. Additionally, a UV LED converts the inputelectrical power into useful UV light more efficiently than a mercury UVlamp by generating less heat and light at unwanted wavelengths, e.g.infrared light. Further, a UV LED has a tighter emission angle than amercury UV lamp, allowing the majority of the UV output to be focuseddirectly where it is required.

In some embodiments, the UV irradiation has a wavelength of 290-430 nm,preferably 315-415 nm, more preferably 330-395 nm, or more preferably350-385 nm, or more preferably 360 nm.

The power of any single UV light source, or the total power of aplurality of the UV light sources, and the number of the UV lightsources may vary widely, depending on the volume of the hydrocarboncontaminated fluid being treated, the initial concentration of thesingle ring aromatic hydrocarbon(s) in the hydrocarbon contaminatedfluid, the type of the UV light source used (e.g. a mercury UV lamp or aUV LED), the way the UV light sources are placed, e.g. the distancebetween the UV light source and a UV irradiation target area of thehydrocarbon contaminated fluid and the carbon nanotubes and thedistribution of the UV light in the fluid and the carbon nanotubes, thesingle ring aromatic hydrocarbon removal efficiency desired, the singlering aromatic hydrocarbon removal efficiency obtainable by theadsorption with the carbon nanotubes alone, the single ring aromatichydrocarbon removal speed, or rate, desired, whether a photocatalyst forUV oxidation and degradation of the single ring aromatic hydrocarbon(s)is present, etc. In some embodiments, the power of the UV light source,preferably a UV LED, is about 10-100 W per liter of the hydrocarboncontaminated fluid, about 20-90 W per liter of the hydrocarboncontaminated fluid, about 30-80 W per liter of the hydrocarboncontaminated fluid, about 40-70 W per liter of the hydrocarboncontaminated fluid, or about 50-60 W per liter of the hydrocarboncontaminated fluid, or preferably about 20 W per liter of thehydrocarbon contaminated fluid.

The amount of the carbon nanotubes per unit volume of the hydrocarboncontaminated fluid to adsorb the single ring aromatic hydrocarbon(s)from the hydrocarbon contaminated fluid can vary, depending on theinitial concentration of the single ring aromatic hydrocarbon(s) in thehydrocarbon contaminated fluid, which typically ranges from 10 mg/L to100 mg/L, the removal efficiency required, the availability of thecarbon nanotubes, the type of the carbon nanotubes used (e.g. unmodifiedor modified carbon nanotubes), the capacity for the treatment of thehydrocarbon contaminated fluid with the carbon nanotubes, e.g. the sizeof a vessel used for batch adsorption, and the capacity for filtrationof the carbon nanotubes from the treated fluid, etc. A typical amount ofthe carbon nanotubes treating the hydrocarbon contaminated fluid rangesfrom about 0.1 g/L to 2 g/L, from 0.15 g/L to 1.5 g/L, from 0.2 g/L to 1g/L, from 0.4 g/L to 0.8 g/L, from 0.5 g/L to 0.7 g/L, of thehydrocarbon contaminated fluid.

In one embodiment, the contacting of the hydrocarbon contaminated fluidwith the carbon nanotubes to adsorb the single ring aromatic hydrocarbonmay be performed in a batch adsorption fashion, for example, by mixingan appropriate amount of the carbon nanotubes with a certain volume ofthe hydrocarbon contaminated fluid based on the ranges of the carbonnanotube amount per unit volume of the hydrocarbon contaminated fluiddescribed above, preferably with the mixture under constant agitation(e.g. stirring, shaking, or vortexing), while exposing the mixture tothe UV irradiation. In some embodiments, the UV light source may beplaced in such a way that it is not in direct contact with thehydrocarbon contaminated fluid/carbon nanotube mixture, for example, theUV light source may be placed on top of, on a side of, and/or underneatha vessel containing the mixture, with the UV light traveling from the UVlight source to reach the mixture, or a portion thereof, preferablyunimpeded, and with the vessel walls preferably made of materialstransparent to the UV light, e.g. UV transparent glass or plastic. Thedistance between the UV light source and a UV irradiation target area ofthe mixture may vary, depending on the desired power level or intensityof the UV light the hydrocarbon contaminated fluid and the carbonnanotubes are exposed to and the desired UV light coverage of thehydrocarbon contaminated fluid and the carbon nanotubes. The UV lightsource may be stationary or may be in motion to increase the UV lightcoverage of the hydrocarbon contaminated fluid and the carbon nanotubes.In other embodiments, the UV light source may be placed in a way that itis in direct contact with the hydrocarbon contaminated fluid/carbonnanotube mixture, for example, the UV light source may be immersed inthe mixture of the hydrocarbon contaminated fluid and the carbonnanotubes. The UV light source may be stationary or may be in motion.These are preferred embodiments when more power of the UV light isdesired to reach and/or penetrate the hydrocarbon contaminated fluid andthe carbon nanotubes.

Besides batch adsorption, granular particles of the carbon nanotubes maybe installed in a fixed bed reactor or fluidized bed reactor. Forexample, the hydrocarbon contaminated fluid containing the single ringaromatic hydrocarbon(s) can be applied to a fixed bed column or reactorof the carbon nanotubes while the hydrocarbon contaminated fluid and thecarbon nanotubes are exposed to the UV irradiation from the UV lightsource, which can be stationary or in motion and which may be placed indirect contact with the hydrocarbon contaminated fluid/carbon nanotubemixture or not, and the effluent of the column or reactor comprises thetreated fluid with reduced concentrations of the single ring aromatichydrocarbon(s). In some embodiments, the fixed bed reactor of the carbonnanotubes comprises a cartridge for easy carry and use. For example,such a cartridge can be attached to a faucet of ground water, orinstalled in a container where (waste) water containing the single ringaromatic hydrocarbon(s) passes through the cartridge from an upper levelof the container, with the treated (waste) water exiting the cartridgeat a lower level of the container with reduced concentrations of thesingle ring aromatic hydrocarbon(s). Further, the batch adsorption, thefixed bed reactor and cartridge, and the fluidized bed reactor describedbelow can include other adsorbents besides carbon nanotubes, such asactivated carbon, graphite, activated alumina, a molecular sieve,aluminophosphate material, silicoaluminophosphate material, zeolites,faujasite, clinoptilolite, mordenite, metal-exchangedsilicoaluminophosphate, uni-polar resin, bi-polar resin, aromaticcross-linked polystyrenic matrix, brominated aromatic matrix, acrylicpolymer, acrylic copolymer, methacrylic polymer, methacrylic copolymer,hydroxyalkyl acrylate, hydroxyalkyl methacrylate, adsorbent carbonaceousmaterial, adsorbent graphitic material, carbon fiber material,nano-material, adsorbent metal salts (including, but not limited toperchlorates, oxalates, and alkaline earth metals), alkaline earth metalmetallic particles, ion exchange resin, linear polymers of glucose,polyacrylamide, or a combination thereof, to adsorb the single ringaromatic hydrocarbon(s), the degradation and oxidation products of thesingle ring aromatic hydrocarbon(s) that may also be toxic, and otherharmful substances in the hydrocarbon contaminated fluid, e.g. heavymetal ions and dyes.

Alternatively, the carbon nanotubes can form a fluidized bed reactorwith the hydrocarbon contaminated fluid containing the single ringaromatic hydrocarbon(s), for example, by introducing the pressurizedhydrocarbon contaminated fluid, either in a liquid form, or in a gaseousform, or in a mixed liquid and gaseous form, through the particulatemedium of the carbon nanotubes. In the fluidized bed reactor, contactbetween the carbon nanotubes and the hydrocarbon contaminated fluid isgreatly enhanced as compared to a fixed bed column or reactor, leadingto a higher adsorption efficiency of the single ring aromatichydrocarbon(s) from the hydrocarbon contaminated fluid. In a fluidizedbed reactor, the UV light source can be likewise stationary or in motionand can be placed in direct contact or not with the hydrocarboncontaminated fluid/carbon nanotube mixture.

The time during which the hydrocarbon contaminated fluid contacts thecarbon nanotubes while the hydrocarbon contaminated fluid and the carbonnanotubes are exposed to the UV irradiation may vary, depending on,without limitation, the adsorption mode (e.g. batch adsorption, fixedbed reactor type, or fluidized reactor type), and the adsorptionconditions (e.g. the agitation speed, the amount and type of the carbonnanotubes, and the temperature and the fluid pH of the batchadsorption), the power and type of the UV light source, and thedistribution and intensity of the UV light the hydrocarbon contaminatedfluid and the carbon nanotubes are exposed to, the initial concentrationof the single ring aromatic hydrocarbon (s) in the hydrocarboncontaminated fluid, the types of the single ring aromatic hydrocarbons,whether photocatalysts are present, the amount and type of thephotocatalysts, and the removal efficiency desired. In some embodiments,the contacting and the exposing time is 5-200 minutes, or 10-180minutes, or 20-150 minutes, or 30-120 minutes, or 40-100 minutes, toachieve a single ring aromatic hydrocarbon removal percentage of atleast 50%, at least 60%, at least 70%, at least 80%, at least 90%, or atleast 95%.

In some embodiments, the hydrocarbon contaminated fluid is an aqueoussolution, the single ring aromatic hydrocarbon is para-xylene, and thereduced concentration of the para-xylene in the treated fluid is atleast 60%, at least 70%, at least 80%, at least 90%, or at least 95%reduced relative to the hydrocarbon contaminated fluid, preferably aftera first 5-60 minutes, or a first 10-50 minutes, or a first 15-40minutes, or a first 20-30 minutes of the contacting and the exposing.

The disclosed method for removing single ring aromatic hydrocarbons froma hydrocarbon contaminated fluid is advantageous in that it is moreefficient and environmentally friendly than removing single ringaromatic hydrocarbons from a hydrocarbon contaminated fluid byadsorption with the carbon nanotubes alone or by UV irradiation alone,not only because of the simultaneous removal of the single ring aromatichydrocarbons by the separate actions of adsorption and UV irradiation,but also because of the collaboration between the two actions. Thesingle ring aromatic hydrocarbon molecules adsorbed by the carbonnanotubes can be subsequently oxidized and degraded by the UVirradiation, making the previously occupied adsorption sites availableagain for new single ring aromatic hydrocarbon molecules and prolongingthe time period of a high adsorption rate. On the other hand, some ofthe oxidation/degradation products of the single ring aromatichydrocarbons that may still be toxic may be adsorbed by the carbonnanotubes, making the treated fluid even more harmless to humans and theenvironment.

In some embodiments, the hydrocarbon contaminated fluid is an aqueoussolution, the at least one single ring aromatic hydrocarbon ispara-xylene, and the reduced concentration of the para-xylene in thetreated aqueous solution is at least about 40% lower, at least about 50%lower, at least about 60% lower, at least about 70% lower, at leastabout 80% lower, or at least about 90% lower, after a first 5-60minutes, or a first 10-50 minutes, or a first 15-40 minutes, or a first20-30 minutes of the contacting and the exposing than that by exposingthe aqueous solution to the UV irradiation for the same amount of timewithout contacting the aqueous solution with the carbon nanotubes toadsorb the para-xylene.

In other embodiments, the hydrocarbon contaminated fluid is an aqueoussolution, the at least one single ring aromatic hydrocarbon ispara-xylene, and the method reduces the concentration of para-xylene inthe aqueous solution by at least 10%, by at least 20%, by at least 30%,by at least 40%, or by at least 50% in less time than a substantiallysimilar method without contacting the aqueous solution with the carbonnanotubes.

In some embodiments, the hydrocarbon contaminated fluid is an aqueoussolution, the at least one single ring aromatic hydrocarbon ispara-xylene, and the reduced concentration of the para-xylene in thetreated aqueous solution is at least about 40% lower, at least about 50%lower, at least about 60% lower, at least about 70% lower, at leastabout 80% lower, or at least about 90% lower after a first 5-60 minutes,or a first 10-50 minutes, or a first 15-40 minutes, or a first 20-30minutes of the contacting and the exposing than that by contacting theaqueous solution with the carbon nanotubes to adsorb the para-xylene forthe same amount of time without exposing the aqueous solution and thecarbon nanotubes to the UV irradiation.

In other embodiments, the hydrocarbon contaminated fluid is an aqueoussolution, the at least one single ring aromatic hydrocarbon ispara-xylene, and the method reduces the concentration of para-xylene inthe aqueous solution by at least 10%, by at least 20%, by at least 30%,by at least 40%, or by at least 50% in less time than a substantiallysimilar method without exposing the aqueous solution and the carbonnanotubes to the UV irradiation.

A plurality of the carbon nanotubes can be pressed, molded, or packagedinto a variety of forms, either alone or in combination with othermaterials (e.g. ABS plastic, a fabric, etc) to facilitate removal of thesingle ring aromatic hydrocarbon(s) from the hydrocarbon contaminatedfluid and/or removal of the carbon nanotubes from the treated fluid whenthe adsorption is complete, and/or when the adsorption capacity of thecarbon nanotubes is exhausted and the carbon nanotubes need to beregenerated. Non-limiting examples of the forms include a granule, apellet, a sphere, a powder, a woven fabric, a non-woven fabric, a mat, afelt, a block, and a honeycomb.

In some embodiments, the contacting forms adsorbed carbon nanotubescomprising at least one single ring aromatic hydrocarbon and the methodfurther comprises removing the adsorbed carbon nanotubes from thetreated fluid. For example, the carbon nanotubes in powder form may beinjected into a storage tank of the hydrocarbon contaminated fluidcontaining the single ring aromatic hydrocarbon(s) and then removed fromthe treated fluid by filtration, centrifugation, or settling. The carbonnanotubes in fiber form may be inserted in a section of the hydrocarboncontaminated fluid (e.g. waste water) treatment piping or trench, and beremoved for regeneration when its adsorption capacity has been exhaustedand replaced by fresh carbon nanotubes. In some embodiments, the carbonnanotubes may comprise a magnetic material, for example, maghemitenanoparticles, as disclosed by Il Tae Kim and Rina Tannenbaum, MagneticCarbon Nanotubes: Synthesis, Characterization and Anisotropic ElectricalProperties, Intech, ISBN 978-953-307-499-3, Published: Jul. 27, 2011(DOI: 10.5772/22636), and incorporated herein by reference in itsentirety, such that the adsorbed carbon nanotubes having the magneticmaterial can be easily removed from the treated fluid with a magnet.

In one embodiment, the method further comprises regenerating theadsorption ability of the adsorbed carbon nanotubes after removing theadsorbed carbon nanotubes from the treated fluid. In some embodiments,the regenerating the adsorption ability of the adsorbed carbon nanotubescomprises heating the adsorbed carbon nanotubes at about 80-300° C.,preferably at about 90-250° C., preferably at about 100-200° C., orpreferably at about 100-150° C., for 10-40 hours, for 15-35 hours, orfor 20-30 hours, to desorb the at least one single ring aromatichydrocarbon. In some embodiments, the carbon nanotubes undergo 1-10cycles, or 2-8 cycles, or 3-7 cycles, or 4-6 cycles, ofregeneration-reuse without a significant loss of the adsorption capacityfor the single ring aromatic hydrocarbon(s).

In another embodiment, the method of removing at least one single ringaromatic hydrocarbon from a hydrocarbon contaminated fluid may take aform of continuous and/or multi-stage adsorption with the carbonnanotubes accompanied by the exposure of the hydrocarbon contaminatedfluid and the carbon nanotubes to the UV irradiation. For example,multiple fixed bed columns or reactors of the carbon nanotubes or, morebroadly, multiple adsorption units of any suitable modes orconfigurations and their combinations, e.g. batch adsorption, fixed bedreactor, cartridge, fluidized bed reactor, etc., can be set up to adsorbthe single ring aromatic hydrocarbon(s) from the hydrocarboncontaminated fluid in a parallel and/or sequential manner. In someembodiments, the adsorption columns, reactors, or units set up in theparallel fashion may be standby columns, reactors, or units ready toreplace another set of parallel columns, reactors, or units whoseadsorption capacity has been exhausted to make the removal operationcontinuous. The carbon nanotubes in replaced columns, reactors, or unitsmay be regenerated and reused. In other embodiments, the adsorptioncolumns, reactors, or units set up in the sequential or serial fashionallow the single ring aromatic hydrocarbon(s) to be adsorbed from thehydrocarbon contaminated fluid through multiple stages to achieve a highremoval efficiency.

The UV oxidation and degradation of the single ring aromatichydrocarbons, such as benzene, toluene, ethylbenzene, and xylene, may beincreased by the presence of H₂O₂ in the hydrocarbon contaminated fluiddue to the conversion of H₂O₂ by the UV irradiation into highly reactivehydroxyl radicals that oxidize and degrade the single ring aromatichydrocarbons, as disclosed by Mahmoud Bahmani, VahidrezaBitarafhaghighi, Kiumars Badr, Peyman Keshavarz, Dariush Mowla, Thephotocatalytic degradation and kinetic analysis of BTEX components inpolluted wastewater by UV/H₂O₂-based advanced oxidation, Desalinationand Water Treatment, Volume 52, Issue 16-18, 2014, incorporated hereinby reference in its entirety. In one embodiment, the fluid ispre-treated with H₂O₂ before contacting the carbon nanotubes and beingexposed to the UV irradiation. In another embodiment, the fluid istreated with H₂O₂ while contacting the carbon nanotubes and beingexposed to the UV irradiation. In still another embodiment, the fluid ispretreated with H₂O₂, and is treated with additional H₂O₂ whilecontacting the carbon nanotubes and being exposed to the UV irradiationto replace the consumed H₂O₂ and keep a H₂O₂ concentration of at least40%, at least 50%, at least 60%, at least 70%, or at least 80% of theinitial H₂O₂ concentration. Since H₂O₂ is thermodynamically unstable anddecomposes to form water and oxygen (2H₂O₂→2H₂O+O₂), with the rate ofdecomposition increasing with rising temperature, concentration and pH,the removal method may be preferably performed at a low ambienttemperature (e.g. 4-30° C., preferably 10-25° C., or preferably 15-20°C.) and/or under a high pressure condition to inhibit H₂O₂ decompositionand/or keep O₂ from escaping from the fluid, e.g. under a pressuregreater than an ambient pressure of about 1 bar, such as 2-100 bar, or5-90 bar, or 10-80 bar, or 15-70 bar, or 20-60 bar, or 30-50 bar. Thefluid may be treated with H₂O₂ by adding a H₂O₂ containing solution tothe fluid, or by pumping or injecting a H₂O₂ containing solution orvapor into the fluid. The initial concentration of the H₂O₂ in thehydrocarbon contaminated fluid may be about 0.2-4 g/L, about 0.4-3 g/L,or about 0.6-2 g/L. The pH of the hydrocarbon contaminated fluid ispreferably in the acidic range of about 1-6, preferably about 2-5, ormore preferably about 3.

In one embodiment, the disclosed method is performed with thehydrocarbon contaminated fluid treated with O₃ or a mixture of O₂ and O₃to increase the UV oxidation and degradation of the single ring aromatichydrocarbons, such as benzene, toluene, ethylbenzene, and xylene, in thefluid through increased production of highly reactive hydroxyl radicals,as disclosed by Katarína Šilhárová, Ján Derco, Peter Tölgyessy, MáriaValičková, Michal Melicher, Reducing of organic petroleum compounds inwater by ozonation/UV processes, 45th International PetroleumConference, Jun. 13, 2011, Bratislava, Slovak Republic, incorporatedherein by reference in its entirety. In some embodiments, O₃ or amixture of O₂ and O₃ is injected into the fluid before and/or while thefluid contacts the carbon nanotubes and is exposed to the UVirradiation. In some embodiments, the concentration of O₃ in thehydrocarbon contaminated fluid is about 0.1-5 mg/L, about 0.2-4.5 mg/L,about 0.5-4 mg/L, about 0.8-3 mg/L, or about 1-2 mg/L.

In the presence of TiO₂ or Mn—TiO₂, a photocatalyst, the UV oxidationand degradation of the single ring aromatic hydrocarbons in thehydrocarbon contaminated fluid may be increased through the productionof superoxide anion radicals (O₂ ⁻.) and hydroxyl radicals that oxidizeand degrade the single ring aromatic hydrocarbons, such as m-xylene, asdisclosed by Yi-Ting Wu, Yi-Hui Yu, Van-Huy Nguyen, Kung-Te Lu, JeffreyChi-Sheng Wu, Luh-Maan Chang, Chi-Wen Kuo, Enhanced xylene removal byphotocatalytic oxidation using fiber-illuminated honeycomb reactor atppb level, Journal of Hazardous Materials 262 (2013) 717-725,incorporated herein by reference in its entirety. In some embodiments,the carbon nanotubes comprise carbon nanotubes coated with TiO₂. Themethods for preparing TiO₂-coated carbon nanotubes have been disclosedby Chinese Patent No. CN101318646 B; by Yuan Yao, Gonghu Li, ShannonCiston, Richard M. Lueptow and Kimberly A. Gray, PhotoreactiveTiO₂/Carbon Nanotube Composites: Synthesis and Reactivity, Environ. Sci.Technol., 2008, 42 (13), pp 4952-4957; and by Ye Cong, Xuanke Li, YunQin, Zhijun Dong, Guanming Yuan, Zhengwei Cui, Xiaojun Lai, Carbon-dopedTiO₂ coating on multiwalled carbon nanotubes with higher visible lightphotocatalytic activity, Applied Catalysis B: Environmental, Volume 107,Issues 1-2, 31 Aug. 2011, Pages 128-134; each incorporated herein byreference in its entirety. In other embodiments, the TiO₂-coated carbonnanotubes may be further processed by an impregnation method in thepresence of a precursor of Mn(NO₃)₂.xH₂O to form a Mn—TiO₂ coating onthe carbon nanotubes. For example, the TiO₂ coated carbon nanotubes maybe soaked in a Mn(NO₃)₂ solution, preferably under sonication for 5-20minutes, and then heated at 60-100° C. to evaporate the Mn(NO₃)₂solution and be dried. The dried carbon nanotubes are subsequentlycalcined in air at a temperature of 300-550° C. for 2-5 hours. In someembodiments, Mn is present at a concentration of 0.5-10 wt %, preferably1-8 wt %, preferably 2-6 wt %, or preferably 3-5 wt %, of the totalweight of the Mn—TiO₂ coating.

In one embodiment, the TiO₂ or Mn—TiO₂ forms a film covering an exteriorsurface and/or an interior surface of the carbon nanotube. In stillanother embodiment, the TiO₂ or Mn—TiO₂ forms a film sandwiched betweensurfaces of the carbon nanotube(s).

The coverage of the surfaces of the carbon nanotubes by the film of TiO₂or Mn—TiO₂ can vary. In some embodiments, the coverage of the surfacesof the carbon nanotubes is about 5-90%, about 10-80%, about 20-60%, orabout 30-50%.

Using the TiO₂ or Mn—TiO₂ coated carbon nanotubes in the disclosedmethod may be advantageous. Firstly, since the carbon nanotubes have alarge surface area, the TiO₂ or Mn—TiO₂ coated carbon nanotubes providea large surface for the coated photocatalyst, enabling the photocatalystto efficiently interact with the UV light to produce the oxidativeradicals oxidizing and degrading the single ring aromatic hydrocarbons.Secondly, the TiO₂ or Mn—TiO₂ coated carbon nanotubes can also adsorbthe single ring aromatic hydrocarbons and/or their oxidation anddegradation products. By changing the loading or coverage of TiO₂ orMn—TiO₂ on the carbon nanotubes, the proportion of the single ringaromatic hydrocarbons removed by UV photocatalytic oxidation versusadsorption by the carbon nanotubes can be varied. The carbon nanotubeswith a relatively low coverage of the photocatalyst coating can removethe single ring aromatic hydrocarbons mainly through adsorption, whereasthe carbon nanotubes with a relatively high coverage of thephotocatalyst coating can remove the single ring aromatic hydrocarbonsmainly through UV photocatalytic oxidation. In one embodiment, thephotocatalyst coated carbon nanotubes with different photocatalystloadings or coverages may be packed in a fixed bed column or reactor insuch way that a photocatalyst loading gradient is formed in theresulting carbon nanotube column or fixed bed reactor along the flowdirection of the hydrocarbon contaminated fluid. In one embodiment, thefluid contacts a section of the carbon nanotube column with a lowerloading of the photocatalyst first and then contacts the subsequentsections of the carbon nanotube column with increasing loadings of thephotocatalyst as it passes through the column with the photocatalystloading gradient. The UV light intensity or power may be adjusted inaccordance with the photocatalyst loading gradient, with more powerdelivered to a column section with a higher photocatalyst loading. UVLEDs are particularly suited to accomplish this, because they have atight emission angle and their UV output can be focused directly towhere it is required. This is a preferred embodiment when the absorptionefficiency and/or rate of the single ring aromatic hydrocarbons with thecarbon nanotubes increase with increasing concentrations of the singlering aromatic hydrocarbons in the hydrocarbon contaminated fluid. Inanother embodiment, the photocatalyst coated carbon nanotubes of a fixedphotocatalyst loading may be mixed with non-photocatalyst coated carbonnanotubes at different ratios to obtain mixed carbon nanotubes withdifferent overall photocatalyst loadings. A similar fixed bed columnwith a photocatalyst loading gradient along the flow direction of thehydrocarbon contaminated fluid can be formed by packing the column withthe mixed carbon nanotubes.

In some embodiments, the hydrocarbon contaminated fluid is contactedwith the carbon nanotubes while the hydrocarbon contaminated fluid andthe carbon nanotubes are exposed to the UV irradiation at a temperatureof about 4-100° C., preferably about 10-90° C., preferably about 15-80°C., preferably about 20-70° C., or preferably about 25-60° C., orpreferably 20-30° C.

In some embodiments, the hydrocarbon contaminated fluid is contactedwith the carbon nanotubes while hydrocarbon contaminated fluid and thecarbon nanotubes are exposed to the UV irradiation at a pressure ofabout 0.1-100 bar, about 0.5-80 bar, preferably about 1-60 bar,preferably about 1-40 bar, preferably about 1-20 bar, or preferablyabout 0.5-1.5 bar.

The concentration of the single ring aromatic hydrocarbons in thehydrocarbon contaminated fluid and/or in the treated fluid can bedetermined by various methods well known in the art, non-limitingexamples of which include GC-MS (See Katarína Šilhárová, Ján Derco,Peter Tölgyessy, Mária Valičková, Michal Melicher, Reducing of organicpetroleum compounds in water by ozonation/UV processes, 45thInternational Petroleum Conference, Jun. 13, 2011, Bratislava, SlovakRepublic, incorporated herein by reference in its entirety), GC/self-ionmolecule reaction (SIMR)/MS/MS (GC/SIMR/MS/MS) (See Chih-Hao Yang,Nadeem Ahmad Khan, Hui-Fen Wu, Differentiation and quantification ofxylene isomers by combining headspace solid-phase microextraction/gaschromatography and self-ion molecule reaction in an ion trap tandem massspectrometry, Journal of Separation Science, Volume 31, Issue 16-17,September 2008, Pages 3050-3057, incorporated herein by reference in itsentirety), GC-FID (See H. Jurdáková, A. Kraus, W. Lorenz, R. Kubinec, Ž.Krkošová, J. Blaško, I. Ostrovský, L. Soják, V. Pacáková, Determinationof gasoline and BTEX in water samples by gas chromatography with directaqueous injection, Petroleum & Coal 47 (3), 49-53, 2005, incorporatedherein by reference in its entirety), and chemometric Raman analysis(See Real-Time Xylene Isomer Quantification Using Chemometric RamanAnalysis from Princeton Instruments, incorporated herein by reference inits entirety).

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified.

EXAMPLE 1 Materials and Methods

1. Materials

Multi-walled carbon nanotubes (CNTs) were purchased from Chengdu OrganicChemicals Co. Ltd with purity of >95% (weight based). Para-xylene(p-xylene) of >99% purity was purchased from Sigma Aldrich and was usedas received without any treatment. The light source used was a UV LEDhaving a wavelength of 365 nm and a power of 20 W.

2. Removal of p-Xylene from an Aqueous Solution

Batch adsorption with carbon nanotubes was used for the removal ofp-xylene. A plurality of beakers was placed on a magnetic stirrer, witheach beaker containing 1000 ml of a freshly prepared p-xylene-watersolution that had an initial p-xylene concentration of 100 ppm and aninitial pH of 6±0.2 under continuous stirring at 600 rpm and at roomtemperature. The beakers containing the p-xylene-water solutions weredivided into three experimental groups. In the first group, 500 mg ofraw CNTs were added to each p-xylene-water solution to adsorb thep-xylene. In the second group, a UV LED was placed on top of each beakercontaining the p-xylene solution to expose the p-xylene solution to UVirradiation. In the third group, 500 mg of raw CNTs were added to eachp-xylene-water solution to adsorb the p-xylene and a UV LED was placedon top of the beaker containing the p-xylene solution/raw CNTs mixtureto expose the p-xylene solution and the raw CNTs to UV irradiation.

The p-xylene solution samples were collected from each experimentalgroup every 30 minutes following the start of the adsorption and/or UVirradiation. The carbon nanotubes were separated from the p-xylenesolution where the carbon nanotubes were present for adsorption bysettling. 1 μL of each p-xylene solution sample was injected into a gaschromatography-flame ionization detector (GC-FID, Agilent 7890B) todetermine the p-xylene concentration in the sample.

The percentage removal of p-xylene was calculated using the followingequation:Removal (%)=C _(o) −C/C _(o)*100  (1)where C_(o) is the initial concentration of p-xylene in thep-xylene-water solution (mg/l) and C represents the concentration (mg/l)of p-xylene in the p-xylene solution samples collected at various timepoints.

EXAMPLE 2

Determination and comparison of p-xylene removal percentages by carbonnanotube adsorption, UV LED irradiation, and a combination of carbonnanotube adsorption and UV

LED Irradiation

1. Removal of p-xylene by Carbon Nanotube Adsorption

Referring to FIG. 1, 500 mg of the multi-walled carbon nanotubes wereadded to 1000 ml of the p-xylene water solution with the initialp-xylene concentration at 100 ppm in the first experimental group, andthe p-xylene concentration in the solution was determined after 30, 60,90, and 120 minutes of adsorption by the carbon nanotubes. After 30minutes of adsorption, about 68% of p-xylene was removed from thesolution. After 120 minutes of adsorption, about 97% of p-xylene wasremoved from the solution. The p-xylene removal or adsorption rate withthe carbon nanotubes was higher at the beginning of the adsorptionprocess and turned lower as the adsorption progressed to an equilibrium,perhaps due to the availability of a large number of vacant adsorptionsites on the surface of the carbon nanotubes at the beginning of theadsorption, and due to the reduced availability or saturation of theadsorption sites on the surface of the carbon nanotubes and therepulsion force between the p-xylene molecules adsorbed on the carbonnanotubes and the free p-xylene molecules remaining in the p-xylenewater solution as the adsorption progressed.

2. Removal of p-xylene by UV Irradiation with a UV LED

Referring to FIG. 1, the p-xylene water solutions with the initialp-xylene concentration at 100 ppm in the second experimental group wereexposed to UV irradiation with a UV LED, and the p-xylene concentrationin the solution was determined after 30, 60, 90, and 120 minutes of UVirradiation. After 30 minutes of the UV irradiation, 81% of p-xylene wasremoved or degraded. After 120 minutes of the UV irradiation, 99% ofp-xylene was removed. The p-xylene removal rate with the UV irradiationwas higher at the beginning of the irradiation process and turned loweras the irradiation process progressed, probably because a large numberof p-xylene molecules were present in the solution and were exposed tothe UV irradiation and degraded at the beginning of the UV irradiationprocess, and as the UV irradiation progressed, a decreasing number ofp-xylene molecules interacted with the UV irradiation and were degraded.

3. Removal of p-xylene by a Combination of Carbon Nanotube Adsorptionand UV LED Irradiation

Referring to FIG. 1, 500 mg of the multi-walled carbon nanotubes wereadded to each of the p-xylene water solutions with the initial p-xyleneconcentration at 100 ppm in the third experimental group to adsorb thep-xylene, while at the same time the p-xylene water solution and themulti-walled carbon nanotubes were exposed to UV irradiation with a UVLED. The p-xylene concentration in the solution was determined after 30,60, 90, and 120 minutes of the combined treatment of the adsorption bythe carbon nanotubes and the UV irradiation. After 30 minutes of thecombined treatment, about 95% of p-xylene was removed from the solution,more than the percentage of p-xylene removed by the adsorption with thecarbon nanotubes alone or by the UV irradiation alone with the UV LEDfor the same amount of time. After 120 minutes of the combinedtreatment, 100% of p-xylene was removed from the solution.

The invention claimed is:
 1. A method of removing at least one singlering aromatic hydrocarbon from a hydrocarbon contaminated fluid,comprising: contacting the hydrocarbon contaminated aqueous solutionwith unmodified multiwalled carbon nanotubes to adsorb the at least onesingle ring aromatic hydrocarbon while exposing the hydrocarboncontaminated aqueous solution and the unmodified multiwalled carbonnanotubes to UV irradiation from at least one LED UV light source with awavelength of about 315-415 nm to form a treated fluid having a reducedconcentration of the at least one single ring aromatic hydrocarbonrelative to the hydrocarbon contaminated aqueous solution, wherein theat least one single ring aromatic hydrocarbon is selected from the groupconsisting of benzene, toluene, ethylbenzene, ortho-xylene, meta-xylene,para-xylene, cumene, and styrene.
 2. The method of claim 1, wherein theat least one single ring aromatic hydrocarbon is para-xylene, andwherein the method reduces the concentration of para-xylene in theaqueous solution by at least 30% in less time than a substantiallysimilar method without contacting the aqueous solution with the carbonnanotubes.
 3. The method of claim 1, wherein the at least one singlering aromatic hydrocarbon is para-xylene, and wherein the method reducesthe concentration of para-xylene in the aqueous solution by at least 30%in less time than a substantially similar method without exposing theaqueous solution and the carbon nanotubes to the UV irradiation.
 4. Themethod of claim 1, wherein the at least one single ring aromatichydrocarbon is para-xylene, and wherein the reduced concentration of thepara-xylene in the treated fluid is at least 95% reduced relative to thehydrocarbon contaminated fluid.
 5. The method of claim 1, wherein theunmodified multiwalled carbon nanotubes have an outer diameter rangingfrom about 8 nm to 80 nm.
 6. The method of claim 1, wherein the amountof the unmodified multiwalled carbon nanotubes contacting thehydrocarbon contaminated aqueous solution ranges from about 0.2 g/L to 1g/L of the hydrocarbon contaminated aqueous solution.
 7. The method ofclaim 1, wherein the hydrocarbon contaminated aqueous solution has a pHof about 6-8 before the contacting and the exposing.
 8. The method ofclaim 1, wherein the hydrocarbon contaminated aqueous solution iscontacted with the unmodified multiwalled carbon nanotubes while thehydrocarbon contaminated aqueous solution and the unmodified multiwalledcarbon nanotubes are exposed to the UV irradiation at a temperature ofabout 20-30° C. and a pressure of about 0.5-1.5 bar.
 9. The method ofclaim 1, wherein a plurality of the unmodified multiwalled carbonnanotubes is in a powder form.
 10. The method of claim 1, wherein thecontacting forms adsorbed unmodified multiwalled carbon nanotubescomprising at least one single ring aromatic hydrocarbon and the methodfurther comprises removing the adsorbed unmodified multiwalled carbonnanotubes from the treated fluid.
 11. The method of claim 10, furthercomprising regenerating the adsorption ability of the adsorbedunmodified multiwalled carbon nanotubes.
 12. The method of claim 1,wherein during the contacting the hydrocarbon contaminated aqueoussolution is stirred with the unmodified multiwalled carbon nanotubes.13. The method of claim 1, wherein during the contacting an hydrocarboncontaminated aqueous solution comprising 100 ppm of the single ringaromatic hydrocarbon is stirred with a suspension of the unmodifiedmultiwalled carbon nanotubes at in ratio of about (500mg unmodifiedmultiwalled carbon nanotubes)/(1,000 ml of the contaminated aqueoussolution).
 14. The method of claim 1, comprising: contacting thehydrocarbon contaminated aqueous solution with the unmodifiedmultiwalled carbon nanotubes for 30 minutes to form a treated fluidcomprising 5 ppm or less of the single ring aromatic hydrocarbon, andcontacting the hydrocarbon contaminated aqueous solution with theunmodified multiwalled carbon nanotubes for 120 minutes to remove all ofthe single ring aromatic hydrocarbon.